Many polymers are used as coatings in controlled release oral pharmaceutical preparations. These polymers are essentially water-insoluble and have consequently low permeability towards water and drugs. If the drug is very soluble and surface of the preparation is large, as in e.g. pellets, low permeability is desired. If the drug is moderately soluble or the surface of the preparation is small, as in e.g. tablets, the permeability of the coating has to be increased.
One method of increasing the permeability of the polymer is disclosed in the U.S. Pat. No. 4,629,619. According to this patent the permeability is increased by including water soluble particles in the coating. When the preparation is subsequently swallowed by a patient and contacted with the GI juices, the water soluble particles dissolve and form pores or channels in the coating through which pores or channels the drug is released from the preparation. The coating including these water soluble pore-forming particles is obtained by a method according to which the water-soluble particles are suspended in an organic solvent. Medical preparations developed according to this patent are currently used and well accepted by the patients. A disadvantage is however the necessity to use organic solvents.
Another method of increasing the permeability is disclosed in the U.S. Pat. Nos. 5,472,712 and 5,639,476. According to these two patents the permeability of the polymer is increased by including a water-soluble material in the polymer. The coating including the water soluble material is obtained by dissolving the water soluble material in the aqueous dispersion of a film-forming polymer. When such a preparation is subsequently swallowed and contacted with the GI juices the water soluble material dissolves and make the polymer coating micro-porous and increase the release rate of the drug through the coating. The sizes of the micro-pores correspond to those of the dissolved molecules. An advantage with medical preparations prepared according to these two patents is that they may be prepared without organic solvent. A disadvantage is that the mechanical strength of the coating is poor, when the permeability is increased to an acceptable level for moderately soluble drugs, especially for tablets. Another disadvantage is that the size of the pores is not controlled which means e.g. that it is difficult to obtain reproducible products.
The permeability increasing or pore-forming agents used according to the two above methods may be identical chemically but the physical state of these agents in the coating solution is different.